This invention relates, in general, to noise reduction circuitry, and more particularly, to noise reduction circuits for an AM (Amplitude Modulated) radio.
An AM radio is suceptible to picking up noise from a number of sources within its environment. For example, an AM radio in an automobile may receive noise from an engine ignition system or an AM radio within a house may receive noise from a florescent light or a light dimmer. The topology of an AM radio receiver circuit exacerbates the noise problem by stretching a noise pulse thereby increasing its duration. Also, a magnitude of a noise pulse may be sufficient to overload an amplifier stage (for example, IF amplifier) in an AM radio signal path.
Noise blanking systems have been employed to reduce noise and prevent overload conditions. An example of such a noise blanking system is described, for example, in "Noise Blanking System For An AM Radio Receiver", U.S. Pat. No. 4,856,084, by Oliver L. Richards, Jr., which patent is hereby incorporated by reference. Noise blanking can be defined as blocking the signal flow of the signal path to "cut out" a noise pulse. In general, noise blanking is achieved by interrupting the signal path or shorting the signal path of an AM radio when a noise pulse is sensed.
Noise blanking systems do not completely remove noise from an AM radio receiver, in other words, some noise is still audible when heard through a speaker (although reduced in magnitude). This is due to the fact that noise blanking systems typically blank for a period of time such that the missing desired modulation in itself creates an audible noise. The predetermined period of time is chosen to blank the high magnitude portion of an average noise pulse, thus not all the noise is removed. A problem with noise blanking systems is the characteristic sound generated by the system. It is believed that the characteristic sound occurs because desired modulation along with the noise pulses are cut out from the incoming signal, thereby interrupting the signal and adding the "characteristic sound".
It would be of great benefit if a noise reduction system for an AM radio could be provided that reduces noise magnitude yet provides a more pleasing and natural characteristic sound.